Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/disassembler_x64.cc

Index: runtime/vm/disassembler_x64.cc
diff --git a/runtime/vm/disassembler_x64.cc b/runtime/vm/disassembler_x64.cc
index 0b8766248d5148c7b9cbd399ae7b1b01ffdd3e6b..dba7e427616823abff399d795765b05d8c9c303e 100644
--- a/runtime/vm/disassembler_x64.cc
+++ b/runtime/vm/disassembler_x64.cc
@@ -798,7 +798,7 @@ static const char* ObjectToCStringNoGC(const Object& obj) {
   const char* full_class_name = clazz.ToCString();
   const char* format = "instance of %s";
   intptr_t len = OS::SNPrint(NULL, 0, format, full_class_name) + 1;
-  char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
+  char* chars = Thread::Current()->zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, format, full_class_name);
   return chars;
 }